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Many of the observed features of zoning in magmatic phenocrysts may be due to the orientation of the section rather than inherent properties of the crystals. An ideal section for the studying of zoning in magmatic crystals has two characteristics: it goes through the center of the crystal, and is perpendicular to one or more crystal faces. Using a model zoned olivine crystal, it is possible to construct accurate zoning profiles for different types of section (centered, symmetrical and skewed). The probability of obtaining a random section which passes within x% of the center of a crystal is shown to be P=0.0x, while the probability that a random section will be within “A” degrees of perpendicular to a given plane is P=sin(A). A systematic approach to the study of zoned crystals is outlined. In particular, it is suggested that composition be plotted against distance cubed, in order to correct for the “volume versus size” problem. A method of determining if a given section goes through (or near) the center of a zoned crystal is also presented. The reasoning in this work applies to other types of magmatic crystals such as pyroxenes and plagioclase.

Although general accounts of carbonatites usually envisage Ca–Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420–460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; εNd=0.2–1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate δ18O=8.0 and δ13C=–7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.

Chemical and isotopic (Sr, O, H) variations have been examined in an iron-rich lava flow of the Kirkpatrick Basalt from the Mesa Range in north Victoria Land, Antarctica. The flow is homogeneous with respect to the less mobile elements, whereas variations observed in K, Na, Si, Fe, and Rb result largely from alteration of glassy matrix material. Whole-rock Rb−Sr isotope data fall along a poorly-defined 103 Ma array attributed to secondary mobilization of Rb during the mid-Cretaceous. Alteration at that time is suggested by paleomagnetic data and would also account for discordant K−Ar dates. Whole-rock δ18O values vary from +5.8 to +8.2‰ and a plagioclase separate has a δ18O value of +5.6‰, reflecting the original composition of the magma. The range of δ18O values for the whole-rock samples results from low-temperature alteration occurring primarily in the Jurassic and/or mid-Cretaceous. Whole-rock δD values (-201 to -243‰) are markedly depleted, approaching equilibrium with modern meteoric water. In light of these data, variable Sr and O isotopic ratios in the underlying sequence of flows, previously interpreted in terms of an assimilation-fractionation model, may largely reflect post-magmatic alteration.